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Decentral Peripherie

Is it necessary to take load into account when connecting to 4-20mA instrument?

Thread Starter: Roger Brandtzaeg   Started: 11/1/2009 8:46 PM   Replies: 2

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  11/1/2009, 8:46 PM
Roger Brandtzaeg
Joined 11/1/2009
Last visit: 11/22/2009
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Hello
Is it necessary to take load into account when connecting to 4-20mA instruments?

We are upgrading an old S5-system, and mainly use these cards:
ET200M
6ES7 331-7TF01-0AB0 (AI 2W,4W)
6ES7 332-8TF01-0AB0 (AO)
ET200ISP
6ES7 134-7TD00 (AI 2W)
6ES7 135-7TD00-0AB0 (AO)

The datasheets says that the cards can supply power to the field instrument as long as the impedance in the loop does not exceed 750ohm.

I have checked the resistance characteristics of more than 1000 field equipments, and all of them were well below 750ohm. (including cable)

So the question is: Is it really necessary to check the resistance of the field instrument when you know that it is 4-20mA ? It looks to me like the resistance of all field instruments follow some sort of industry standard/understanding.
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  11/6/2009, 9:55 PM
edr
Joined 12/22/2006
Last visit: 11/21/2009
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Hello,
if the load is too high, the voltage needed to be able to reach 20 mA would be too high. For a board suited for both 10Volt and 4-20mA, a typical value is 500 Ohm (20 mA over 500 Ohm is 10 Volt).
Best regards
EDR
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  11/8/2009, 1:48 PM
Roger Brandtzaeg
Joined 11/1/2009
Last visit: 11/22/2009
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Thanks for the reply
But the thing I am really wondering about is wheter these resistance values follow some sort of standard. If all PLCs are made to work with roughly a maxiumum of 750ohm, then I would think all instrument suppliers would make their equipment accordingly... Consequently there would be no need for me to check the resistance characteristics of the equipment.
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